Noble gas leak detectors for detecting infinitesimally small leaks by means of inert noble tracer gases such as helium, argon, neon and the like, preferably helium, are known. One such detector is shown in U.S. Pat. Nos. 3,070,992 and 3,227,872. The leak detector disclosed in those patents has a mechanical pump and a diffusion pump to evacuate the system, a pressure gauge tube, a mass spectrometer to detect gaseous material and a getter pump to remove active gases during the leak checking process.
The leak detector of those patents uses a titanium sublimation getter pump to remove the active gases and to hopefully leave the noble gases for leak detection. However, helium may be entrapped under the titanium that is sublimed off of a filament, which allows helium later to evolve slowly. That slow evolution of helium causes undesirable helium background and false signals.
Other aspects of these devices also contribute to an unwanted helium background with the result that these devices could only repeatedly detect leaks larger than, 10.sup.-12 atm cc/sec at 20.degree. C.
Other helium leak detectors use diffusion pumps that exhaust the gas from the diffusion pump into a mechanical pump. This gas then goes to the atmosphere. A liquid nitrogen cold trap is placed above the diffusion pump to capture oil molecules backstreaming from the diffusion pump and to also capture water vapor. A turbomolecular pump is sometimes used to replace the diffusion pump and liquid nitrogen trap. An inlet system on the other side of the mass spectrometer chamber allows gases to enter from the device under test. The inlet system contains a series of valves and usually another mechanical pump for initial evacuation of the gases in the device under test along with the inlet system.
Bergquist U.S. Pat. No. 4,492,110 entitled "Ultra Sensitive Noble Gas Leak Detector" uses a non-evaporable getter pump in the vacuum system that contains the mass spectrometer. During the leak test period no gases are exhausted. Active gases are captured by the getter pump, noble gases are not. The noble gases remain in a gaseous state and if helium enters the system through a leak in the component, the helium pressure increases. The helium pressure increases as long as helium surrounds the leak. This increase is detected and measured by the mass spectrometer, which detects leak rates as small as 10.sup.-16 torr liter/sec at 20.degree. C. However, the partial pressure of the noble gas remains uniform throughout the vacuum system. If the system has a very large volume, the partial pressure of the tracer gas will be too low to be detected even after a long period of accumulation.
Bergquist U.S. Pat. No. 4,608,866 entitled "Small Component Helium Leak Detector" uses a modified cryopump which entraps all gases except helium on its cooled surfaces. Helium remains in a gaseous state inside the vacuum chamber. This system entraps the gases entering from the inlet, not the exhaust side of the turbomolecular pump. Also, as with U.S. Pat. No. 4,492,110, there is no mechanism to increase the pressure of the tracer gas.
Bergquist U.S. Pat. No. 4,785,666 entitled "Method of Increasing the Sensitivity of Leak Detector in the Probe Mode" uses a modified cryopump, but again on the inlet side and without a tracer gas enricher.
The difference between Bergquist U.S. Pat. No. 4,608,866 and U.S. Pat. No. 4,785,666 is the approach used to introduce the gases into the cryopump. In U.S. Pat. No. 4,608,866 the gas comes in and out of a baffled flange at the top of the cryopump. Not all gases that are on top of the pump immediately enter the pump. Some of the gases may also come out of the cryopump before they enter the cold region and are frozen. U.S. Pat. No. 4,785,666 has the inlet gases going in on the bottom side of the cryopump where most of the gases are entrapped except helium which goes out on the top side of the pump to the leak detector to be measured.
Japanese U.S. Pat. No. 59-48630 aims at preventing the backstreaming of atmospheric helium to the detector region. It uses a sorption pump and a "helium reservoir" instead of a mechanical exhaust pump during the test cycle. This may eliminate the back diffusion of helium from the atmosphere and through the mechanical exhaust pump. However, the partial pressure of helium at the sensor head remains very low. The detector senses helium as it flows by the sensor head on its way to the "helium reservoir" and the sorption pump.
Japanese U.S. Pat. No. 58-35434 measures the helium pressure at the mid point of a turbomolecular pump, but does not allow the helium to accumulate nor does it preferentially pump active gases to lower the total pressure. The total pressure in the vacuum chamber must be approximately 10.sup.-10 torr before making the test, therefore leak testing is limited to extremely low chamber pressures.
None of these prior art devices uses an entrainment pump placed on the exhaust side of a turbomolecular pump, removing the active gases and collecting the noble gases, thus reducing the active gases to low enough pressure that a mass spectrometer can be operated to measure the noble gases accumulated in or around the entrainment pump.